1. Field of the Invention
The present invention relates generally to a voltage regulator for a sensor and, more particularly, to a regulator circuit that is connectable in series between an AC power source and a load and provides a means for monitoring both the current and voltage of the circuit in association with a means for controlling the current in response to both the current and voltage monitoring means.
2. Description of the Prior Art
It is well known to those skilled in the art that a voltage regulator circuit can be provided for connection in an electrical circuit in series with an AC power supply and a load. For example, voltage regulators of this type are used to provide DC power for a sensor such as a photoelectric sensor or a proximity detector. When the sensor is not in a condition under which the load is intended to receive power from the power supply, the regulator circuit passes only a minimal current through the load sufficient to power the sensor. When the sensor, on the other hand, senses a status under which the load is to be provided with electrical power from the power supply, the voltage regulator circuit connects the load to the power supply.
U.S. Pat. No. 4,219,803, which issued to Kuwabara et al on Aug. 26, 1980, describes a two-wire fire sensing and receiving system. The system has a plurality of fire sensors connected in parallel in which the difference in current level between a feeding electric circuit of a relatively small level for controlling the system and a sensing current of a relatively high level for causing the switching element of an alarm device to be conductive is maintained to distinguish the current levels by restraining the current which tends to increase at the beginning of capacitor charging and at intermittent charging periods within the level of the feeding electric current without being affected by the fluxuation in power supply voltage.
U.S. Pat. No. 4,205,327, which issued to Dahlke on May 27, 1980, discloses a two-wire current transmitter. The transmitter is intended to control the total current in the two-wires in accordance with a value of a parameter to be sensed. The transmitter provides for substantial linearization of the electrical characteristic representative of the value of the parameter to be sensed by adjusting the current from an adjustable current control to substantially compensate for the nonlinear relationship of the parameter to be sensed versus the electrical output of the sensor of the parameter. The circuit also provides for substantial compensation for sensor lead wire effects.
U.S. Pat. No. 4,142,219, which issued to Kuwabara et al on Feb. 27, 1979, discloses a two-wire system that includes a signal receiving section and a detection section with a protected relay. The system includes a power source, a signal receiving section and a sensing device in series which supplies electric power to and receives sensing signals from signaling devices in the system. A capacitor is connected to each sensing device for preventing a voltage drop upon initiation of its sensing action. A buffer circuit is further coupled with the receiving section for protecting against false operation by an initial current which increases upon switching the power source, thus insuring that the system becomes stable.
U.S. Pat. No. 4,250,490, which issued to Dahlke on Feb. 10, 1981, describes a two-wire transmitter for converting a varying signal from a remote sensor to a DC current signal. The transmitter provides a signal representative of a parameter measured by a variable reactance sensor which may be located in a hostile environment. The sensor may also be remote from the signal conditioning and transmitter electronics. Transmission circuitry from the electronics includes a separate transformer winding energized by a pair of twisted cables to reduce the capacitive, inductive and resistive effects of long wires. The transformer circuitry provides power to the sensor which provides the signal conditioning and transmitter electronics a signal representing the parameter measured. The system permits the modification of the total current through the two wires used to carry power to the signal conditioning and transmitter electronics so that the total current is representative of the measured parameter.
U.S. Pat. No. 4,271,448, which issued to Pond on Jun. 2, 1981, discloses an electronic protection circuit for solid state switches energizing AC loads. The two-terminal circuit of the fold-back type is inserted in electrical series relationship with an AC load and an associated load energizing, solid state, semiconductor switch. Under normal operating conditions, the transistor circuit presents a very low impedance to permit load energization when the solid state switch is conducting and hence has a very low impedance. Upon sensing an overcurrent condition, caused for example by a short circuit across the load, the transistor circuit rapidly switches to and locks into a high impedance circuit protection state to limit current through the series associated solid state switch. A charged capacitor holds the transistor circuit into its high impedance state subsequent to clearing of the short circuit fault until reset by interrupting the application of power to it. The disclosed protection scheme finds particularly advantageous use in the short circuit protection of an AC line operated proximity switch.
U.S. Pat. No. 4,329,730, which issued to Ryczek et al on May 11, 1982, discloses a DC switch circuit applicable for a switch which receives an input from a sensor head such as a proximity sensor, limit switch or other type of condition responsive sensor. In its most complex form, the circuit includes a power supply/regulator, an output load current source, an output load current sink, input logic to receive the signal from the sensing head, a selector to determine whether the source and sink will be normally opened or normally closed, and a device for checking the outputs for faults and indicating the presence of such.
As the technology relating to sensors advances, the demand for significantly reduced size and cost of the sensors also increases. As the size of sensors decreases, the size of the voltage regulator circuit used in association with the sensing circuitry must also decrease. It would therefore be beneficial if a voltage regulator circuit for a sensing device can minimize the number of high power components and utilize individual components for more than one purpose within the circuit. The reduction of high power components in the voltage regulator circuit permits the regulator to be enclosed within a housing of reduced size and, in addition, reduces the overall cost of the circuit.